[RFC PATCH 09/10] uaccess: Refuse and warn on more user accesses without an mm

[James Houghton <jthoughton@google.com>]

Several generic user access paths don't go through the copy helpers
that check for a NULL current->mm. Add the uaccess_without_mm() check to
them as well, so that they warn and fail without attempting the access:

 - __scoped_user_access_begin(), covering scoped_user_*_access() and
   get_user_inline()/put_user_inline() on all architectures, including
   when masked user access is used.
 - strnlen_user().
 - check_zeroed_user().
 - The iov_iter user copy routines that call into architecture-specific
   helpers: copy_to_user_iter_mc() (copy_mc_to_user()),
   copy_from_user_iter_nocache() (copy_from_user_inatomic_nontemporal()),
   copy_from_user_iter_flushcache() (copy_from_user_flushcache()) and
   zero_to_user_iter() (clear_user()).

Only check for a NULL mm here rather than using uaccess_should_fail(),
so as not to start injecting failures in these paths.

Paths implemented entirely by architectures, e.g. get_user(),
put_user(), clear_user(), and direct users of user_access_begin() or
masked_user_access_begin(), are still not covered.

Signed-off-by: James Houghton <jthoughton@google.com>
---
 include/linux/uaccess.h | 2 ++
 lib/iov_iter.c          | 8 ++++++++
 lib/strnlen_user.c      | 3 +++
 lib/usercopy.c          | 3 +++
 4 files changed, 16 insertions(+)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index eae204f7261b..7d26d2de9599 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -781,6 +781,8 @@ static __always_inline void __scoped_user_rw_access_end(const void *p)
 ({									\
 	typeof(uptr) __retptr;						\
 									\
+	if (uaccess_without_mm())					\
+		goto elbl;						\
 	if (can_do_masked_user_access()) {				\
 		__retptr = masked_user_##mode##_access_begin(uptr);	\
 	} else {							\
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 21e2ccf1e8f2..a0769874241c 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -204,6 +204,8 @@ static __always_inline
 size_t copy_to_user_iter_mc(void __user *iter_to, size_t progress,
 			    size_t len, void *from, void *priv2)
 {
+	if (uaccess_without_mm())
+		return len;
 	if (access_ok(iter_to, len)) {
 		from += progress;
 		instrument_copy_to_user(iter_to, from, len);
@@ -277,6 +279,8 @@ static __always_inline
 size_t copy_from_user_iter_nocache(void __user *iter_from, size_t progress,
 				   size_t len, void *to, void *priv2)
 {
+	if (uaccess_without_mm())
+		return len;
 	return copy_from_user_inatomic_nontemporal(to + progress, iter_from, len);
 }
 
@@ -296,6 +300,8 @@ static __always_inline
 size_t copy_from_user_iter_flushcache(void __user *iter_from, size_t progress,
 				      size_t len, void *to, void *priv2)
 {
+	if (uaccess_without_mm())
+		return len;
 	return copy_from_user_flushcache(to + progress, iter_from, len);
 }
 
@@ -451,6 +457,8 @@ static __always_inline
 size_t zero_to_user_iter(void __user *iter_to, size_t progress,
 			 size_t len, void *priv, void *priv2)
 {
+	if (uaccess_without_mm())
+		return len;
 	return clear_user(iter_to, len);
 }
 
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index 4a6574b67f82..45709798eb8f 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -96,6 +96,9 @@ long strnlen_user(const char __user *str, long count)
 	if (unlikely(count <= 0))
 		return 0;
 
+	if (uaccess_without_mm())
+		return 0;
+
 	if (can_do_masked_user_access()) {
 		long retval;
 
diff --git a/lib/usercopy.c b/lib/usercopy.c
index e2f0bf104a59..9146c729c47a 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -48,6 +48,9 @@ int check_zeroed_user(const void __user *from, size_t size)
 	if (unlikely(size == 0))
 		return 1;
 
+	if (uaccess_without_mm())
+		return -EFAULT;
+
 	from -= align;
 	size += align;
 
-- 
2.56.0.307.g5fe7bd680c-goog